Turn it into a ratio.
The beginning ratio is 3:20, because there are 3 counselors for every 20 campers.
Then, you do x:120, x representing the number of counselors.
You have to multipy 20 by 6 to get 120, so you multiply 3 by 6 to get x.
Therefore, there are 18 counselors if there are 120 campers.